In general, a lesion portion shows a color different from the color of a normal mucosal tissue. In accordance with improvement of performance of a color endoscope device in recent years, it has become possible for an operator to conduct diagnosis by visually recognizing a lesion portion having a color only slightly different from the color of a normal tissue. However, in order for the operator to conduct diagnosis by visually recognizing a lesion portion precisely from a normal tissue in accordance with a slight color difference on a captured image obtained by an endoscope, the operator needs to be trained for a long period under guidance of an expert. Even for an expert, it is not easy to conduct diagnosis by visually recognizing a lesion portion based on a slight color difference, and therefore cautious work is required in the diagnosis.
In view of the above, for example, Japanese Patent Provisional Publication No. 2014-18332A (hereafter, referred to as patent document 1) describes an apparatus configured to execute scoring for a lesion portion on a captured image so as to assist diagnosis of the lesion portion by an operator. Specifically, the apparatus described in the patent document 1 executes a tone enhancement process in which a nonlinear gain is applied to pixel values of pixels constituting a captured image obtained by an endoscope so as to enhance a dynamic range in a portion around a boundary of a region including pixel values judged to be the lesion portion. The apparatus subsequently converts tone-enhanced pixel data in an RGB space defined by R, G and B primary colors into a predetermined color space, such as an HIS color space or an HSV color space, to obtain information on hue and saturation, judges whether each pixel is a pixel of the lesion portion based on the information on hue and saturation, and calculates an evaluation value (a lesion index) based on the number of pixels judged to be the lesion portion.